Cooperative feedback system and method for a compensation system associated with a transmitter or codec

ABSTRACT

A cooperative feedback system is provided for a compensation system associated with, for example, a transmitter or codes, for enabling the compensation system to improve the accuracy of digital signals transmitted to a digital network. The cooperative feedback system is particularly suited for providing feedback to a compensation system for correcting distortion resulting from rob bit signaling (RBS), digital loss, or other types of digital signal degradation. The cooperative feedback system includes a compensation selector in a transmitter (e.g., digital modem, analog modem, codec, etc.) that combines compensations with frames of digital data by way of an addition mechanism to produce modified digital data frames. The transmitter is configured to transmit the modified digital data frames into the digital network. A receiver (e.g., digital modem, analog modem, etc.) is configured to receive the modified digital data frames from the digital network, to determine if accuracy of each of the modified digital data frames has increased based upon a corresponding compensation, and to communicate one or more quality feedback signals to the transmitter via the digital network. The one or more quality feedback signals indicate whether the accuracy of each of the digital data frames has increased based upon the corresponding compensations. The transmitter receives the quality feedback signal(s) and selects and implements the compensation that yields a highest accuracy based upon the quality feedback signal(s).

This application is a continuation of application Ser. No. 08/697,371filed Aug. 23, 1996 now U.S. Pat. No. 5,754,594.

FIELD OF THE INVENTION

The present invention generally relates to data communications and, moreparticularly, to a cooperative feedback system and method for acompensation system associated with, for example, a transmitter orcodec, for enabling the compensation system to improve the accuracy ofdigital signals transmitted to a digital network. The cooperativefeedback system and method are particularly suited for providingfeedback to a compensation system for correcting distortion resultingfrom rob bit signaling (RBS) and/or digital loss.

BACKGROUND OF THE INVENTION

A telephone network is often used as an interface between a digitalmodem and an analog modem. Generally, a digital modem is a device thatcommunicates digital data by using digital signals that replicate analogwaveforms. An analog modem is a device that communicates digital data byencoding the data on analog waveforms.

FIG. 1 shows a typical telephone network 99 for interconnecting adigital modem 101 and an analog modem 102. The digital modem 101 isusually interconnected with a digital network 113 via digitalconnections 112a, 112b. For instance, the digital modem 101 may beinterconnected to a digital network 113 in the form of a public switchtelephone network (PSTN) via a Local Exchange Carrier (LEC) subscriberloop. The digital network 113 may comprise, among other things, a T1carrier system, a basic rate or primary rate Integrated Services DigitalNetwork (ISDN), a fiber optic cable network, a coaxial cable network, asatellite network, or even a wireless digital communications network.Communications over the digital network 113 are conducted in accordancewith a pulse code modulation (PCM) scheme. Channel capacity throughthese digital facilities is typically between 56 and 64 kilobits persecond (kb/s). Coding of the signals is also employed so thatcompression and a constant signal/distortion performance over a widedynamic range is achieved for optimal transmission of voice signals.

A commonly used coding technique is a nonlinear mu-law coding. Thelinear amplitude L corresponding with each code word is encoded, orconverted to, a mu-law code word by the following equation:

    M=(L+33)*2.sup.8-N

where M is the mu-law magnitude (e.g., 4 bits), L is the linearamplitude (e.g., 14 bits), and N is the mu-law segment, or frame (e.g.,3 bits). The mu-law code word is decoded, or converted to, a linear codeword as follows:

    L={(2M+33)2.sup.N -33}

The digital network 113 is in turn interconnected with another LECsubscriber loop that includes a coder/decoder (codec) 106. The codec 106is interconnected with the digital network 113 via digital connections114a, 114b. The codec 106 is often situated at a telephone companyoffice or along a street near the analog modem subscriber in asubscriber loop carrier (SLC) device. The codec 106 provides aninterface between the digital network 113 and an analog telephoneconnection 118, sometimes referred to as a copper loop. Forcommunications in the direction from the digital network 113 to theanalog modem 102, the codec 106 includes a mu-to-lineardigital-to-analog converter (DAC) 109. The converter 109 convertsnonlinear mu-law levels to a linear analog signal.

For communications in the direction from the analog modem 102 to thedigital network 113, the codec 106 includes a linear-to-muanalog-to-digital (ADC) converter 107. The converter 107 converts thelinear analog signal to nonlinear mu-law code words.

A hybrid 103 is in communication with the DAC and ADC via respective lowpass filters (LPFs) 111, 105. The hybrid 103 serves to separate thebidirectional analog signals from the analog telephone connection 118into unidirectional transmit and receive analog signals sent to andreceived from the ADC 107 and the DAC 109, respectively.

Furthermore, the analog modem 102 is connected to the analog telephoneconnection 118 and communicates analog signals therewith. Thus,communications occur between the digital modem 101 and the analog modem102 by way of the digital network 113 and the codec 106.

A method known as rob bit signaling (RBS) is oftentimes utilized in thedigital network 113 to communicate on-hook/off-hook status between themodems 101, 102 and the digital network 113. RBS forces the leastsignificant bit (LSB) of every nth frame, where n is typically 6 or 24,to a constant logic level, either logical 1 or 0. Unfortunately, RBScauses the block error rate of data transfers to increase and the peakerror to increase from 0.5 LSB to 1.5 LSB, as is illustrated in FIG. 2.

More specifically, with reference to FIG. 2, logic states are encoded bya transmit subsystem associated with the digital modem 101 in accordancewith the encode step function indicated at reference numeral 122. Thepossible maximum error e resulting from the encoding/decoding processfor any given signal level is 0.5 LSB. Further, when an RBS frameoccurs, the LSB is driven to a predetermined logical state, either amark (logical 1) or a space (logical 0). Therefore, the logic states aredecoded as indicated by step functions 123 and 124 for the mark andspace, respectively, as shown FIG. 2. The possible maximum error in boththe 1-RBS frame and the 0-RBS frame resulting from the encoding/decodingprocess is 3*e, or 1.5 LSB. Almost needless to say, modem performance isseriously degraded as a result of RBS.

Hence, there exists a need in the industry for systems and methods forcoping with RBS and for increasing the speed of data transfers throughthe digital network that periodically robs a bit.

SUMMARY OF THE INVENTION

The invention provides for a cooperative feedback system and method fora compensation system associated with, for example, a transmitter orcodec, for enabling the compensation system to improve the accuracy ofdigital signals transmitted to a digital network. The cooperativefeedback system and method of the invention are particularly suited forproviding feedback to a compensation system for correcting distortionresulting from rob bit signaling (RBS), digital loss, or other types ofdigital signal degradation.

The invention can also be conceptualized as providing a method forenabling improvement of a digital signal transmitted to a digitalnetwork that periodically robs a bit. This method can be broadlysummarized as follows: combining at a transmitter (e.g., analog modem,digital modem, codec, etc.) different compensations with respectiveframes of digital data to produce modified digital data frames;communicating the modified digital data frames from the transmitter to areceiver (e.g., analog modem, digital modem, etc.); determining at thereceiver if the accuracy of each of the digital data frames is increasedbased upon a corresponding compensation; transmitting from the receiverto the transmitter a quality feedback signal for each of the modifieddigital data frames indicative of whether the corresponding compensationhas increased the accuracy; receiving the quality feedback signals atthe transmitter; and selecting at the transmitter one of thecompensations that yields a highest accuracy based upon the qualityfeedback signals.

For purposes of clarity and simplicity and in no way limited thereto,the cooperative feedback system and method of the invention will bediscussed hereafter in connection with a rob bit compensation system. Inthe context of RBS, the cooperative feedback system enables a rob bitcompensation system to improve the accuracy of digital signalstransmitted to a digital network, such as a telephone network, whichperiodically robs a bit every nth frame, where n is, for example but notlimited to, 6, 12, or 24, by providing the rob bit compensation systemwith information regarding which frames (RBS frames) have a bit robbedtherefrom. As a result of the inventive system and method, high speeddata transfers through the digital network are realized.

The rob bit compensation system can be implemented, among other places,within the transmit subsystem of a modem, which transfers digital datato the digital network, or alternatively, in a coder/decoder (codec) inconnection with the data path leading to the network. The rob bitcompensation system is configured to combine a compensation with eachRBS frame of digital data in order to enhance the accuracy of datatransfers. As an example of a compensation, a quantity, such as one-halfof an LSB of the frame, may be added to or subtracted from each RBSframe.

The rob bit compensation system can also be implemented in acoder/decoder (codec). In this configuration, the system is associatedwith the data path leading to the digital network.

In general, the cooperative feedback system advises the rob bitcompensation system (in the transmitter or codec) as to which frames ofoutgoing digital data are to ultimately have a bit robbed therefrom bythe digital network. The cooperative feedback system includes acompensation selector that causes different compensations to be combinedwith frames of digital data by way of an addition mechanism to producemodified digital data frames. The transmit subsystem or codec isconfigured to transmit the modified digital data frames at respectivetime intervals into the digital network.

A receive subsystem associated with, for example, a remote modem, isconfigured to receive the modified digital data frames from the digitalnetwork, to determine if accuracy of each of the modified digital dataframes has increased based upon a corresponding compensation, and tocommunicate one or more quality feedback signals to the transmitsubsystem via the digital network. The one or more quality feedbacksignals indicate whether the accuracy of each of the digital data frameshas increased based upon the corresponding compensations.

The compensation selector in the transmit subsystem or codec isconfigured to receive the quality feedback signal(s). In turn, theselector selects and implements the compensation that yields a lowesterror based upon the quality feedback signal(s).

The invention has numerous advantages, a few of which are delineatedhereafter, as merely examples.

An advantage of the invention is that it enables the rob bitcompensation system to increase the accuracy of data transmitted to adigital network.

Another advantage of the invention is that data transfer rates throughthe digital network can be increased.

Another advantage of the invention is that, in the context of RBS, theinvention enables the rob bit compensation system to reduce the RBSinduced peak error in a signal that is passed through a digital networkthat practices RBS from 1.5 LSB to 1.0 LSB.

Another advantage of the invention is that, in the context of RBS, theinvention can detect any number and frequency of RBS frames, even whenthe digital network includes a plurality of subnetworks, each of whichrobs its own bit.

Another advantage of the invention is that it is simple in design,easily implemented with existing modems, and is reliable in operation.

Another advantage of the invention is that it can be implemented withsoftware, hardware, or a combination thereof. Other objects, features,and advantages of the present invention will become apparent to one withskill in the art upon examination of the following drawings and detaileddescription. It is intended that all such additional objects, features,and advantages be included herein within the scope of the presentinvention, as defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings. The components in the drawings are not necessarily to scale,emphasis instead being placed upon clearly illustrating principles ofthe present invention. In the drawings, like reference numeralsdesignate corresponding parts throughout the several views.

FIG. 1 is an electronic block diagram of a possible implementation forcoupling together digital and analog modems over a digital network;

FIG. 2 is a graph comparing the effects of rob bit signaling (RBS) in aprior art decoding system and in a decoding system that employs the robbit compensation system;

FIG. 3 is an electronic block diagram of a possible implementation oftransmit subsystem that employs the rob bit compensation system of FIG.3 within the digital modem of FIG. 1;

FIG. 4 is an electronic block diagram of a possible implementation of acodec that employs the rob bit compensation system of FIG. 3;

FIG. 5 is electronic block diagram of a possible first embodiment of afeedback system (cooperative) associated with the compensation selectorof FIG. 5; and

FIG. 6 is electronic block diagram of a possible second embodiment of afeedback system (noncooperative) associated with the compensationselector of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A rob bit compensation system 130 can be implemented in connection witha data path in either the digital modem 101 (FIG. 1) or the codec 106(FIG. 1) in order to compensate for and minimize the distortion imposedupon data that is transmitted to a digital network 113 (FIG. 1) thatperiodically robs a bit from every nth frame, where n is typically 6,12, or 24. When implemented in the digital modem 101, the rob bitcompensation system 130 can be employed in connection with a transmitsubsystem (FIG. 3). When implemented in the codec 106 (FIG. 4), the robbit compensation system 130 can be employed in connection with the datapath that passes data to the digital network 113.

Note the commonly assigned U.S. Pat. No. 5,761,247 entitled "Rob BitCompensation System and Method Associated With A Receiver Or Codec,"filed Aug. 23, 1996, by the inventors herein describes a rob bitcompensation system that can be utilized in connection with a receivesubsystem or codec for manipulating data that is received from thedigital network, as opposed to data that is transmitted to the digitalnetwork.

A. Transmit Subsystem

FIG. 3 is an electronic block diagram of a possible implementation of atransmit subsystem 181 that can be situated within the digital modem 101(FIG. 1) for receiving transmit data 183 from a local DTE andcommunicating it to the digital network 113 (FIG. 1). The transmitsubsystem 181 can employ the rob bit compensation system 130 in order tominimize distortion that will ultimately be imposed upon the transmitdata by the digital network 113 when the digital network 113prospectively robs a bit(s).

Referring to FIG. 3, the transmit subsystem 181 communicates thetransmit data 183 in the form of a binary bit stream from the local DTEto an encoder 189. The encoder 189 modulates and/or processes thetransmit data 183. For this purpose, the encoder 189 may implement, forexample, a serial-to-parallel converter, a filter, a scrambler, atrellis encoder, a modulator, etc. The encoder 189 can employ anysuitable modulation and/or signal processing techniques, for example,those recommended by the conventional V.34 standard. When the encoder189 employs V.34 modulation and signal processing, the data 126 that isoutput from the encoder 189 complies with the well known V.34 protocolso that the data stream corresponds with one of the fourteen possibleV.34 speeds between 2400 b/s, and 33,600 b/s, inclusive.

The encoder 189 drives the linear digital signal 126 to the rob bitcompensation system 130, which is configured to compensate for RBSframes, when RBS frames are detected, while passing frames unchanged inthe case of non-RBS frames. The transmit subsystem 181 of FIG. 3 cannotdirectly detect RBS LSBs from the transmit signal because the RBSprocessing does not occur until a later time in the digital network 113.Accordingly, the transmit subsystem 181 utilizes a feedback system 210,240 (FIGS. 5, 6) that employs a feedback for detecting RBS frames andfor advising the compensation control 133 in the rob bit compensationsystem 130 of the same. The feedback system 210, 240 can be designed inmany different manners. As mere examples, first and second embodimentsof a feedback system 210, 240 are shown in respective FIGS. 5 and 6 andare described in detail hereinafter.

For purposes of simplicity at this point in the discussion, it will beassumed that the rob bit compensation system 130 receives feedback inthe form of space and mark initialization signals 153a, 153b,respectively, from another component in the feedback system 210, 240,indicating when RBS frames occur. Based upon this information, thecompensation control 133 of the rob bit compensation system 130 detectswhen compensation is appropriate and provides a compensation quantity tobe added to each RBS frame when the RBS frame is present. In thepreferred embodiment, during an RBS frame, the compensation additionmechanism 131 causes one-half of an LSB to be added to the lineardigital code word 126 corresponding with an anticipated RBS frame whenthe R13S LSB is a space (logical 0), or alternatively, one-half of anLSB to be subtracted from (i.e., add 2's complement to) the lineardigital code word 126 corresponding with an anticipated RBS frame whenthe RBS LSB is a mark (logical 1). The foregoing is set forthmathematically hereafter.

Non-RBS Frames

For frames of digital data that are not to have a rob bit, the rob bitcompensation system 130 passes the digital data essentially unchanged tothe linear-to-mu converter 197, which converts the linear code words tomu-law code words via the following equation:

    M=(L+33)*2.sup.8-N

RBS Frames Having A Mark LSB

For frames of digital data that are to have a rob bit in the form of amark, the combination of the rob bit compensation system 130 andlinear-to-mu converter 197 functions as follows. The linear amplitude Lcorresponding with each code word is converted to a mu-law code word bythe following equation:

    M=(L+32)*2.sup.8-N

RBS Frames Having A Space LSB

For frames of digital data that are to have a rob bit in the form of aspace, the combination of the rob bit compensation system 130 andlinear-to-mu converter 197 functions as follows. The linear amplitude Lcorresponding with each code word is converted to a mu-law code word bythe following equation:

    M=(L+34)*2.sup.8-N

Architecturally, the compensation addition mechanism 131 as shown inFIG. 3 includes successive adders 131a, 131b and is controlled by acompensation control 133. Generally, the compensation addition mechanism131 receives successive 8-bit linear code words 126 from the encoder 189and to each, it performs one of the following: (a) mathematicallycombines a zero with the code word 126 to convert the 8-bit code word126 to a 9-bit code word 137 (i.e., effectively, forwards the code word126 unchanged but with an additional bit); (b) adds 0.5 LSB to the codeword 126 in order to produce a 9-bit code word 137; or (c) subtracts 0.5LSB (i.e., adds 2's complement of 0.5 LSB, or -0.5 LSB) to the code word126 in order to produce a 9-bit code word 137. In order to accomplishthe aforementioned functionality, the adder 131a of the compensationaddition mechanism 131 is configured to add either nothing or +0.5 LSBto each 8-bit code word 126 in order to generate a 9-bit code word 127,while similarly, the adder 131b is configured to add either nothing or-0.5 LSB to each 9-bit code word 127 in order to generate a 9-bit codeword 137.

The compensation control 133 controls the adders 131a, 131b, asindicated by respective reference arrows 151, 161, to accomplish one ofthe aforementioned options. Furthermore, the compensation control 133 isconfigured to receive the space and mark initialization signals 153a,153b, which are essentially a bit pattern that identifies which framesare RBS frames. Based upon the foregoing initialization signals 153a,153b, the compensation control 133 detects when an RBS frame occurs andis configured to advise the compensation addition mechanism 131 when tocombine a quantity (i.e., to effectively add or subtract an amount) withthe RBS frame in order to enhance the accuracy of the RBS frame.

In architecture, the compensation control 133 of the rob bitcompensation system 130 has mark RBS logic for identifying an RBS framehaving an LSB that is a mark and space RBS logic for identifying an RBSframe having an LSB that is a space. The space and mark RBS logicsgenerate respective control signals 151, 161 for the compensationaddition mechanism 131. The signals 151, 161 are indicative of whetheror not there is an RBS frame and whether the RBS frame either has aspace LSB or a mark LSB, respectively.

The mark RBS logic includes a mark ring counter 146. The mark ringcounter 146 is a shift register or other suitable mechanism. The markring counter 146 is configured to receive its own output at its dataterminal (D). The mark ring counter 146 is preset so that all statesmatch the mark initialization word 153b and is clocked by a clock signal155 at a rate of preferably 8000 hz to correspond with the framefrequency rate of the incoming signal 126.

In the preferred embodiment, the mark ring counter 146 is an n statedevice, where n represents the frequency of the RBS frame. Typically, nis 6, 12, or 24. The mark ring counter 146 is shifted one state of nstates every frame. In FIG. 3, the mark ring counter 146 shifts to theleft so that the most significant bit (MSB) is output at reference arrow148. When RBS compensation should be applied, the mark ring counteroutput (i.e., the most significant bit (MSB) of the stored word)exhibits a logical 1, and automatically tracks the RBS frames that havea mark LSB.

The space RBS logic identifies RBS frames having an LSB that exhibits aspace. The space RBS logic includes a space ring counter 156. The spacering counter 156 is a shift register or other suitable mechanism. Thespace ring counter 156 is configured to receive its own output at itsdata terminal (D). The space ring counter 156 is preset so that allstates match the space initialization (INIT) word 153a and is clocked bya clock signal 155 at a rate of preferably 8000 hz to correspond withthe frame frequency rate of the incoming digital data 126.

In the preferred embodiment, just as with the mark ring counter 146, thespace ring counter 156 is an n state device, where n represents thefrequency of the RBS frame, and the space ring counter 156 is shiftedone of n states every frame. In FIG. 3, the space ring counter 156shifts to the left so that the MSB is output at reference arrow 158.When space RBS compensation should be applied, the space ring counter156 output (i.e., the most significant bit (MSB) of the stored word)exhibits a logical 1 and automatically tracks the RBS frames that have aspace LSB.

As a result of the rob bit compensation system 130 in FIG. 3, thetransmit subsystem 181 can encode each RBS frame so that the maximumpossible error in each RBS frame at a receiver is no greater than 1.0LSB (i.e., 2*e), as is reflected at reference numerals 123a, 124a inFIG. 2. Recall that the possible maximum error in the RBS frameresulting from the encoding/decoding process of the prior art is 1.5 LSB(i.e., 3*e). Almost needless to say, modem performance is significantlyimproved by the invention during an RBS frame.

Further note that multiple RBS bits can occur as the signal passesthrough multiple switches, multiplexers, or subscriber loop carriers(SLC) associated with the digital network 113. The ring counters 146,156 can compensate for RBS in multiple bit positions.

The linear-to-mu converter 197 converts each linear digital code word137 to a mu-law nonlinear digital code word 198, which is output to thedigital network 113 (FIG. 1) via a switch 203, which operates atpreferably 8000 hz. The connection 112b is typically a T1 or ISDNconnection, which operates with a frame switching frequency of 8000 hz.Moreover, the digital network 113 typically uses multiplexers that usemu-law encoded 64 kb/s PCM for transmission.

As a result of the rob bit compensation system 130 in FIG. 3, thetransmit subsystem 181 can encode the RBS frame so that the error in theRBS frame at the receiver is no greater than 1.0 LSB.

Note that the elements of the transmit subsystem 181 of FIG. 3, asdescribed previously, can be implemented in software, firmware,hardware, or a combination thereof. In the preferred embodiment, theseelements, and particularly, the rob bit compensation system 130, isimplemented in software that is stored in memory and that configures anddrives a digital signal processor (DSP). Furthermore, the rob bitcompensation system 130 can be stored on any computer-readable mediumfor use by or in connection with a computer-related system or method.

EXAMPLE

In order to further clarify operation of the rob bit compensation system130, an example with specific data is set forth hereafter. First, it isassumed that the rob bit compensation system 130 receives data 126 inthe form of a series of 8-bit frames with bit patterns as set forth inTable A hereafter.

                  TABLE A    ______________________________________           FRAME #                  DATA LSB    ______________________________________           1      0110000 1           2      0010111 0           3      1111001 1           4      1010010 1           5      0000100 0           6      0101011 1           1      1111000 1           2      1010000 0    ______________________________________

Based upon the frames set forth in Table A, the feedback system 210, 240(FIGS. 5 and 6) will determine that the digital network 113 employs 3rob bits in the form of a mark in the frames 1, 3, and 4.

Accordingly, the mark ring counter 146 is loaded with the markinitialization word 153b (FIG. 3) of "101100", each bit of whichcorresponds to a frame, and the space ring counter 156 is loaded with aspace initialization word 153a (FIG. 3) of "000000", each bit of whichcorresponds to a frame. The words 153a, 153b are shifted one bit to theleft in each counter 156, 146 during each frame, and the mostsignificant bit (MSB) in each counter 146, 156 is analyzed by beingpassed to the respective adders 131a, 131b, as indicated bycorresponding reference arrows 151, 161 (FIG. 3).

When the MSB in mark ring counter 146 is a logical "1", then the adder131b will add -0.5 LSB to the respective frame, and in converse, whenthe MSB in the mark ring counter 146 is a logical "0", then the adder131b will add nothing to the respective frame. Similarly, when the MSBin the space ring counter 156 is a logical "1", then the adder 131a willadd +0.5 LSB to the respective frame, and in converse, when the MSB inthe space ring counter 156 is a logical "0", then the adder 131a willadd nothing to the respective frame.

As is apparent from the examples of the bit patterns, the mark ringcounter 146 will cause -0.5 LSB to be added to select frames, while thespace ring counter 156 will cause no quantities to be added to any ofthe frames. Table B set forth hereafter illustrates the shifting of theword in the mark ring counter 146 and the implementation of compensationduring RBS frames.

                  TABLE B    ______________________________________                      MARK RING    FRAME # DATA LSB  COUNTER VALUE                                   COMPENSATION    ______________________________________    1       0110000 1 101100       yes    2       0010111 0 011001       no    3       1111001 1 110010       yes    4       1010010 1 100101       yes    5       0000100 0 001011       no    6       0101011 1 010110       no    1       1111000 1 101100       yes    2       1010000 0 011001       no    ______________________________________

As illustrated in Table B, with the specific counter bit patterns setforth previously in this example, every first, third, and fourth framewill be compensated by addition of -0.5 LSB, while the remainder of theframes are left unchanged, regardless of the current state of the LSB inthat frame.

B. Coder/Decoder (Codec)

The codec 106 (FIG. 1) can also be equipped with a rob bit compensationsystem 130 in connection with one or both of its data streams. The robbit compensation system 130 may be associated with the communicationsconnection 114b leading from the digital network 113. Commonly assignedU.S. Pat. No. 5,761,297 entitled "Rob Bit Compensation System AssociatedWith A Receiver Or Codec," filed Aug. 23, 1996, by the inventors hereindescribes a codec having a rob bit compensation system 130 associatedwith the latter communications path. The rob bit compensation system 130may also be associated with the communications connection 114a leadingto the digital network 113, as is described hereafter. However, thisimplementation generally requires some type of feedback in order to knowwhich frames are LSB frames.

With reference to FIG. 4, in regard to the communications connection114a, the rob bit compensation system 130 is configured to receive thestream of mu-law nonlinear digital code words as indicated by referencearrow 114a from the linear-to-mu ADC 107 in the codec 106. Further, thesystem and method 130 is adapted to combine compensation, whenappropriate, to produce a compensated stream 114a' of mu-law nonlineardigital code words for the digital network 113.

In the preferred embodiment, the rob bit compensation system 130 causesone-half of an LSB to be added to each mu-law nonlinear digital codeword 114b corresponding with the RBS frame when the RBS LSB is a space(logical 0), or alternatively, one-half of an LSB to be subtracted fromeach mu-law nonlinear digital code word 114b corresponding with the RBSframe when the RBS LSB is a mark (logical 1).

Note that the elements of the codec 106 of FIG. 4 can be implemented insoftware, firmware, hardware, or a combination thereof. In the preferredembodiment, the elements 103, 105, 107, 109, and 111 are implemented inhardware, while the rob bit compensation system 130 is implemented inhardware or software/firmware in the ADC 107. In embodiments where thesystem 130 is implemented in software/firmware, it may be stored andtransported on any computer-readable medium for use by or in connectionwith a computer-related system or method.

C. First Embodiment of a Feedback System

The first embodiment is a cooperative feedback system and method,wherein elements of a transmitter cooperate with elements in a receiverin order to identify distorted digital data frames. The methodology ofthe cooperative feedback system can be broadly summarized as follows:combining at a transmitter (e.g., analog modem, digital modem, codec,etc.) different compensations with respective frames of digital data toproduce modified digital data frames; communicating the modified digitaldata frames from the transmitter to a receiver (e.g., analog modem,digital modem, etc.); determining at the receiver if the accuracy ofeach of the digital data frames is increased based upon a correspondingcompensation; transmitting from the receiver to the transmitter aquality feedback signal for each of the modified digital data framesindicative of whether the corresponding compensation has increased theaccuracy; receiving the quality feedback signals at the transmitter; andselecting at the transmitter one of the compensations that yields ahighest accuracy based upon the quality feedback signals.

For purposes of clarity and simplicity and in no way limited thereto,the cooperative feedback system will be discussed hereafter inconnection with the rob bit compensation system 130. In the context ofRBS, the cooperative feedback system enables the rob bit compensationsystem 130 to improve the accuracy of digital signals transmitted to thedigital network 113 by providing the rob bit compensation system 130with information regarding which frames (RBS frames) have a bit robbedtherefrom.

FIG. 5 is an electronic block diagram of the cooperative feedbacksystem, generally denoted by reference numeral 210, that can be utilizedto provide feedback regarding RBS frames to the compensation control 133(FIG. 3). In this example, at the start of a call, a compensation isinserted and then the response of the remote modem (e.g., modem 102 inFIG. 1) is monitored for the existence of performance improvement. Ifperformance improves, then the compensated frame is considered an RBSframe. The foregoing process is continued at the start of the call for apredetermined number n of frames.

The feedback system 210 includes elements in the local transmitsubsystem 181 associated with, for example, the modem 101 (FIG. 1),cooperating with elements in the remote receive subsystem 214 associatedwith, for example, the analog modem 102 (FIG. 1).

The elements situated at the local transmit subsystem 181 include thefollowing. An OR logic gate 205 is configured to generate a selectorcontrol signal(s) 206, based upon receipt of an initialization signal(s)207 from the modem controller or of a feedback signal(s) 112a from theremote receive subsystem 214. The initialization signal 207 is producedinitially in order to commence the cooperative feedback system 210 totest compensation(s). In general, the signal 206 includes informationregarding the time when a compensation system 130 should be activatedand its compensation setting, and the signal 112a includes informationregarding which of the compensations resulted in better performance.

A compensation selector 204 is configured to receive the selectorcontrol signal 206. When the compensation selector 204 is advised totest compensations, the compensation selector 204 switches to differentcompensations at predetermined time intervals. In the preferredembodiment, the various compensations are implemented and tested byreconfiguring the mark and space ring counters 146, 156 (FIG. 3) duringeach test trial, as conceptually illustrated in FIG. 5 via select andinitialization signals 208, 153a, 153b leading to each of n differentcompensations and OR logic 212 connected to each of the compensations asindicated by reference arrows 211.

Note that the n compensations can be any type, but in the context ofcorrecting RBS, each compensation is generally the addition orsubtraction of 0.5 LSB to a frame.

During each test trial, the selected compensation 213 is passed onto theaddition mechanism 131 (adders 131a, 131b in FIG. 3), which combines thecompensation 213 with a startup sequence 218 associated with the localtransmit subsystem 181. In the local transmit subsystem 181, either thetransmit data 183 (also, FIG. 3) or the aforementioned startup sequence218 is communicated to the addition mechanism 131, as indicated via ORlogic 217. An example of a startup sequence would be the probe sequencedescribed in the V.34 specification. The V.34 probe sequence could bepartitioned into 24 consecutive segments. The V.34 probe sequence lastsapproximately 300 milliseconds. With this implementation, thecompensation 213 would be changed every 13 milliseconds.

During each time interval, the addition mechanism 131 provides acompensated frame 225 to the linear-to-mu converter 197. Further, theconverter 197 outputs the 8-bit mu-law code words 198 to the switch 203,which in turn communicates them to digital network 113 via connection112b at 8000 hz.

The remote receive subsystem 214 receives the mu-law code wordscorresponding to the compensated frames in succession from theconnection 118 via a switch 229 at 8000 hz. The mu-law code words 231from the switch 229 are passed to standard signal processing components,including a mu-to-linear converter, a decoder, etc., as denotedcollectively by block 232. The decoder generally demodulates andprocesses the incoming linear code words in accordance with any suitableformat, such as the V.34 protocol. A comparator 234 receives the decodedbit stream 235 from the block 232 of components and compares it with areference 236. In the preferred embodiment, the reference 236 isessentially a bit stream that is free of errors and that should havebeen received by the remote receive subsystem 214. However, in otherpossible embodiments, the signal 235 could be a signal-to-noise level,in which case, the reference 236 would be a signal-to-noise thresholdlevel.

A good/bad indicator 238, such as a bit, to indicate whether or not theaccuracy of signal 235 has improved is generated by the comparator 234for each frame and forwarded to a feedback transmitter 239, whichperforms any suitable processing of the good/bad indicators 238. Thecollection of good/bad indicators 238 or a derivative signal thereof isreturned to the compensation selector 204 of the local transmitsubsystem 181 via the feedback transmitter 239 situated at the remotereceive subsystem 214, and the connection 118. The indicators 238 may beforwarded together in a single segment, independently, or in groups backto the transmit subsystem, whichever is appropriate.

In the preferred embodiment, the remote receive subsystem 214 analyzeseach segment independently, reporting back to the local transmitsubsystem 181 through the MP Type 1 sequence, which is a portion of thetraining sequence specified by the V.34 recommendation. Using currentlyundefined or reserved bits in the MP sequence, an indication as to whichframes were improved is relayed back to the local transmit subsystem181.

With the good/bad indicators 238 from the receive subsystem 214, thecompensation selector 204 of the transmit subsystem 181 can apply thebest overall compensation 214 to allow high speed operation. Multiplerob bits may be compensated.

Additionally, during V.34 renegotiations in fast retrains, informationor remote performance quality information can be fed back to thetransmitter 212. Examples of information fed back could be, but notlimited to, overall signal quality, signal-to-noise ratio, receivesignal level, etc.

In response to the good/bad indicators 238, the compensation selector204 determines which frames are RBS frames. Based upon thisdetermination, the compensation selector 204 programs the mark and spacering counters 146, 156 (FIG. 3) to implement a particular compensation.

D. Second Embodiment of a Feedback System

The second embodiment is a noncooperative feedback system and method,wherein elements of a transmitter do not cooperate with elements in areceiver in order to identify distorted digital data frames. Themethodology of the noncooperative feedback system can be broadlysummarized as follows: combining at a transmitter (e.g., analog modem,digital modem, codec, etc.) a plurality of compensations to segments ofdigital data respectively to produce modified digital data segments;communicating the modified digital data segments from the transmitter toa receiver (e.g., analog modem, digital modem, etc.); receivingretransmissions requests at the transmitter from the receiver;determining which of the compensations yields a lowest error based uponthe retransmission requests; and implementing the lowest errorcompensation at the transmitter.

For purposes of clarity and simplicity and in no way limited thereto,the noncooperative feedback system and method will be discussedhereafter in connection with the rob bit compensation system 130. In thecontext of RBS, the noncooperative feedback system enables the rob bitcompensation system 130 to improve the accuracy of digital signalstransmitted to a digital network 113 by providing the rob bitcompensation system 130 with information regarding which frames (RBSframes) have a bit robbed therefrom.

FIGS. 6 is electronic block diagram of the noncooperative feedbacksystem, generally denoted by reference numeral 240. In this example ofan implementation, a method for detecting RBS frames involves monitoringrequests for block retransmission from the remote modem 102 (FIG. 1).This method can be used, for example, with any modem that uses standardV.42 error correction. An advantage of this method is that it does notrequire modification of any existing modem and does not requirecooperative effort between the transmitting and receiving modems.

More specifically, referring to FIG. 6, the noncooperative feedbacksystem 240 includes elements in the local transmit subsystem 181corresponding with those shown in FIG. 4 and described previously. Forpurposes of simplicity, the discussion relating to the components of thelocal transmit subsystem 181 of FIG. 4 is incorporated herein byreference.

In the noncooperative feedback system 240 of FIG. 6, the local transmitsubsystem 181 compensates the RBS frames one at a time with thecompensations 209. After each compensation 209 in a particular frame,the transmit subsystem 181 will dwell for a predetermined time periodwhile the local receive subsystem 121 monitors the channel 112a forretransmission requests. The receive subsystem 121 can be equipped withany suitable error correction mechanism 242 for monitoringretransmission requests. In the preferred embodiment, the errorcorrection mechanism 242 is a standard V.42 error correction mechanismset forth in the industry standard V.42 specification. For RBS,compensating the correct frame will result in higher data rates anddecreased retransmission requests. Compensating on the wrong frame willresult in lower data rates and increased retransmission requests. Basedupon the number of retransmission requests, the error correctionmechanism 242 determines which frames are RBS frames and generates thefeedback control signal 193, which is communicated to the compensationselector 204.

Based upon the feedback 193 from the error correction mechanism 242, thecompensation selector 204 can select and implement a compensation(s), ifappropriate, via programming the mark and space ring counters 146, 156(FIG. 3).

An advantage of the noncooperative feedback system 240 is that no lossof data synchronization ever occurs. The system 240 also works with anyvender's product that supports automatic rate control, V.42 bis errorcorrection, or other error correcting protocols.

D. Software

The rob bit compensation system 130 can be implemented in software. Onepossible implementation of the software version for the transmitsubsystem 181 is set forth hereafter.

    ______________________________________    lin2mu:    a0=a0>>1    a0=a0>>1    /* right justify 14 bits */    a0=rnd(a0) /* round 14 bit value */    a0=a0<<1    /* left justify 14 bits */    a0=a0<<1    /* a0h = Sddd dddd dddd dd00 */    y=0x8000   /* mask for sign bit */            /* y = 1000 0000 0000 0000 */    a1=a0&y    /* a1h = S000 0000 0000 0000 */    if ne a0=-a0               /* if negative take 2s complement */            /* input data in-place in a0h */    /* Begin Digital Loss Compensation */    r0=d.sub.-- loss1               /* Get loss scaler table pointer */    pt=*r0    y=a0 x=*pt++    d.sub.-- lossin:    p=x*y *r3=a1    a1=p    y=132      /* y = 0000 0000 1000 0100 */            /* bias value (33) shifted left by 2 */            /* bits to compensate for the 14 bit */            /* input data being left justified */    a1=a1+y    /* a0h += 0000 0000 1000 0100 */            /* *linmu = S000 0000 0000 0000 */    c1=-8      /* initialize counter 1 for finding */            /* the segment */    do 8 {         /* determine the segment number */    ifc p1 a1=a1<<1                 /* (exponent) and the */    }          /* compressed code word into place */               /* a0h = 1ccc cxxx xxxx xxxx */    a1=a1>>8    x=a1        /* Temp store of a1 data */    a1=c2       /* Move c2 to c0 */    c0=a1    c1=-1       /* Set c1 for no RBS */    rd=d.sub.-- loss2                 /* Read RBS indentifier bits */    a1=*r0    a1=a1<<1      /* Shift next bit to msb */    *r0=a1      /* Save RBS indentifier */    a1=x        /* Restore a1 */    if p1 goto lin2mu05                 /* No changes if msb=0 */    a1=*r0    y=0x200     /* RBS identifier bit */    a1=a1|y                /* Reset RBS identifier */    *r0=a1      /* Save RBS identifier */    a1=x        /* Restore a1 */    y=4    a1=a1+y    y=0x80      /* Test for negative result */    a1&y    if ne goto lin2mu05                 /* Jump if no RBS needed */    c1=0       /* Set c0 for RBS */    lin2mu05:    y=0x78    a1=a1&y    y=132      /* put 1s before and after the */    a1=a1+y     /* compressed code word */            /* a1h = 0000 0000 001c ccc1 */    if c1ge a1=a1<<1                 /* Shift if RBS */    do 7 {        /* the formatted compressed */    if c01t a1=a1<<1                 /* code word into place, i.e. */    }         /* apply exponent (segment) */              /* a1h = 00 0000 001c ccc1 seg = 0 */              /* a1h = 00 0000 01cc cc10 seg = 1 */              /* a1h = 00 0000 1ccc c100 seg = 2 */              /* a1h = 00 0001 cccc 1000 seg = 3 */              /* a1h = 00 001c ccc1 0000 seg = 4 */              /* a1h = 00 01cc cc10 0000 seg = 5 */              /* a1h = 00 1ccc c100 0000 seg = 6 */              /* a1h = 01 cccc 1000 0000 seg = 7 */    a1=a1-y      /* a1h -= 0x21 substract bias (33) */    y=a1 x=*pt++    p=x*y    a0=p    /* End Digital Loss Compensation */    /* End */    y=132       /* y = 0000 0000 1000 0100 */            /* bias value (33) shifted left by 2 */            /* bits to compensate for the 14 bit */            /* input data being left justified */    a0=a0+y     /* a0h += 0000 0000 1000 0100 */            /* *linmu = S000 0000 0000 0000 */    c1=-8      /* initialize counter 1 for finding */            /* the segment */    do 8 {         /* determine the segment number */    ifc p1 a0=a0<<1                 /* (exponent) and the */    }         /* compressed code word into place */              /* a0h = 1ccc cxxx xxxx xxxx */    a0=a0>>4     /* the compressed code word */    a0=a0<<1     /* into place */            /* a0h = 1111 cccc xxxx xxxx */    a1=c2      /* c2 = negative segment number */    a1=-a1     /* a1h = segment number */            /* a1h = 0000 0000 0000 0eee */    a1=a1<<4     /* segment number into place */    a1=a1<<8     /* a1h = 0eee 0000 0000 0000 */    y=0x0f00    /* mask for compressed code word *            /* y = 0000 1111 0000 0000 */    a0=a0&y     /* a0h = 0000 cccc 0000 0000 */    y=a1       /* y = 0eee 0000 0000 0000 */    a0=a0+y y=*r3                 /* add segment number to a0h */            /* a0h = 0eee cccc 0000 0000 */            /* y = S000 0000 0000 0000 */    a0=a0+y     /* add sign bit to a0h */            /* a0h = Seee cccc 0000 0000 */    a0=a0>>8     /* to low byte of a0h */            /* a0h = xxxx xxxx Seee cccc */    y=0x00ff    /* mask for 8 bit Mulaw value */    a0=a0&y      /* mask 8-bits to insure removal of */            /* sign extension */            /* a0h = 0000 0000 Seee cccc */    a0= a0 y    /* invert bits to provide an inverted*/            /* Mulaw value in low byte of a0h */    return    d.sub.-- losstb1:    int 1.0, 1.0, 0.5, 0x7fff, 0.7071, 1.4142    END OF CODE    ______________________________________

Many variations and modifications may be made to the preferredembodiments of the invention. All such modifications and variations areintended to be included herein within the scope of the presentinvention, as is defined by the following claims.

In the claims set forth hereinafter, the structures, materials, acts,and equivalents of all "means" elements and "logic" elements areintended to include any structures, materials, or acts for performingthe functions specified in connection with said elements.

Wherefore, the following is claimed:
 1. A system for enablingimprovement of a digital signal transmitted through a digital network,comprisingsaid digital network that periodically robs a bit; atransmitter configured to combine compensations with frames of digitaldata to produce modified digital data frames, said transmitterconfigured to transmit said modified digital data frames into saiddigital network, a receiver configured to receive said modified digitaldata frames from said digital network, said receiver configured todetermine if accuracy of each of said modified digital data frames hasincreased based upon a corresponding compensation, said receiverconfigured to communicate to said transmitter via said digital network aquality feedback signal indicative of whether said accuracy of each ofsaid digital data frames has increased based upon said correspondingcompensations; said transmitter configured to receive said qualityfeedback signal and to select and implement which of said compensationsyields a highest accuracy based upon said quality feedback signalswherein said transmitter comprises:a mark ring counter configured toreceive a mark initialization signal, said mark ring counter configuredto generate a mark ring counter signal during each clocked shift of saidmark ring counter; space ring counter configured to receive a spaceinitialization signal, said space ring counter configured to generate aspace ring counter signal during each clocked shift of said space ringcounter; a mark adder configured to mathematically combine a quantitywith said digital data based upon said mark ring counter signal; and aspace adder configured to mathematically combine a quantity with saiddigital data based upon said space ring counter signal.
 2. A method forenabling improvement of a digital signal transmitted to a digitalnetwork, comprising the steps of:combining at a transmitter differentcompensations with respective frames of digital data to produce modifieddigital data frames; communicating said modified digital data framesfrom said transmitter to a receiver; determining at said receiver ifsaid accuracy of each of said digital data frames is increased basedupon a corresponding compensation; transmitting from said receiver tosaid transmitter a quality feedback signal for each of said modifieddigital data frames indicative of whether said correspondingcompensation has increased said accuracy; receiving said qualityfeedback signals at said transmitter; and selecting at said transmitterone of said compensations that yields a highest accuracy based upon saidquality feedback signals.
 3. The method of claim 2, further comprisingthe steps of:converting said digital data from linear digital data tomu-law digital data; and communicating said mu-law digital data to saiddigital network.
 4. The method of claim 2, further comprising the stepof modifying every nth frame of said digital data after said selectingstep, where n is a multiple of six.
 5. A cooperative feedback method,comprising the steps of:receiving digital data; determining whether afirst frame of said digital data is a rob bit frame by comparing saidfirst frame of digital data to a reference; and transmitting a qualityfeedback signal indicating whether said first frame is a rob bit frame,said quality feedback signal based on said determining step.
 6. Themethod of claim 5, further comprising the steps of:receiving saidquality feedback signal; and compensating a second frame of digital databased on said quality feedback signal.
 7. A cooperative feedback system,comprising a receiver configured to receive a first frame of digitaldata, to compare said first frame of digital data to a reference inorder to determine whether said first frame is a rob bit frame, and totransmit a quality feedback signal indicating whether said first frameis said rob bit frame.
 8. The system of claim 7, wherein said qualityfeedback signal is configured to be received by a transmitter configuredto compensate a second frame of digital data based on said qualityfeedback signal.
 9. The system of claim 8, wherein said transmitter isfurther configured to process said quality feedback signal and todetermine, based on said quality feedback signal, a pattern of frameshaving a bit robbed therefrom.
 10. The system of claim 7, wherein saidquality feedback signal is based on a comparison by said receiver ofsaid first frame of said digital data to said reference.
 11. The systemof claim 7, wherein said receiver is further configured to receive saidfirst frame of digital data via an analog connection.
 12. The system ofclaim 11, wherein said receiver is further configured to transmit saidfeedback signal across said analog connection.
 13. A cooperativefeedback system, comprising a receiver for receiving a first frame ofdigital data and for providing a quality feedback signal indicatingwhether said first frame is distorted by rob bit signalling.
 14. Thesystem of claim 13, wherein said receiver is configured to compare saidfirst frame of digital data to a reference in order to determine whethersaid first frame is distorted.
 15. The system of claim 13, wherein saidquality feedback signal is configured to be received by a transmitter,said transmitter configured to compensate a second frame of digital datafor distortion from said rob bit signalling by combining a quantity tosaid second frame.
 16. The system of claim 13, wherein said first frameof digital data is encoded on an analog waveform when said receiverreceives said first frame of digital data.
 17. A cooperative feedbackmethod, comprising the steps of:receiving a first frame of digital datafrom a digital network via an analog waveform; determining whether saidfirst frame is an RBS frame; and transmitting a quality feedback signalto enable compensation of distortion due to rob bit signalling.
 18. Themethod of claim 14, further comprising the step of compensating a secondframe of digital data based on said quality feedback signal.
 19. Themethod of claim 18, wherein said compensating step includes the step ofcombining a quantity with said second frame of digital data.
 20. Themethod of claim 17, wherein said determining step further includes thestep of comparing said first frame to a reference.
 21. A cooperativefeedback receiver for improving accuracy of digital data received from adigital network that induces distortion from rob bit signalling (RBS),said receiver comprising:a means for receiving a first frame of saiddigital data encoded on an analog waveform; a means for decoding saidfirst frame of digital data from said analog waveform and fordetermining whether said first frame of said digital data is an RBSframe; and a means for transmitting a quality feedback signal to enablecompensation of a second frame of digital data for errors associatedwith RBS.
 22. The receiver of claim 21, wherein said quality feedbacksignal is configured to be received by a transmitter that compensates asecond flame of digital data based on said quality feedback signal. 23.The method of claim 22, wherein said transmitter compesnates said secondframe of digital data by combining a quantity to said second frame. 24.A cooperative feedback receiver configured to receive digital data, todetermine whether said digital data is distorted from rob bit signalling(RBS), and to generate a quality feedback signal indicating whether saiddigital data is distorted by said RBS.
 25. The receiver of claim 24,wherein said quality feedback signal is configured to be received by atransmitter that combines a compensation quantity to a frame of digitaldata in order to reduce distortion from said RBS.
 26. The receiver ofclaim 24, wherein said receiver is coupled to an analog connection andwherein said receiver receives said digital data via said analogconnection.
 27. A cooperative feedback method, comprising the stepsof:receiving digital data having a bit forced to a particular logiclevel by a digital network; determining whether said digital networkforced said bit to said particular logic level; and generating a qualityfeedback signal at an analog modem for enabling compensation ofdistortion associated with said digital network forcing bits of data tosaid particular logic level.
 28. The method of claim 27, furthercomprising the steps of:receiving said quality feedback signal; andcompensating for said distortion based on said quality feedback signal.29. The method of claim 28, wherein said compensating step includes thestep of combining a quantity to a frame of digital data.
 30. Acooperative feedback method for improving the accuracy of digital datatransmitted through a digital network, comprising the steps of:receivinga quality feedback signal indicating whether a frame of digital data issubject to distortion from said digital network forcing at least one bitof said frame to a constant logic level, said quality feedback signaltransmitted from an analog modem; and compensating said frame of digitaldata for said distortion based on said quality feedback signal.
 31. Themethod of claim 30, wherein said compensating step includes the step ofcombining a quantity to said frame of digital data.
 32. A cooperativefeedback method for improving the accuracy of digital data transmittedthrough a digital network, comprising the steps of:receiving a qualityfeedback signal indicating whether a frame of digital data is subject todistortion from said digital network forcing at least one bit of saidframe to a constant logic level; and compensating said frame of digitaldata based on said quality feedback signal, wherein said compensatingstep includes the step of combining a quantity to said frame of digitaldata and wherein said quantity is 0.5 of a least significant bit.
 33. Acooperative feedback method, comprising the steps of:receiving a qualityfeedback signal indicating whether a frame of digital data is subject todistortion from rob bit signalling, said quality feedback signaltransmitted from an analog modem; and compensating said frame of digitaldata when said frame is subject to said distortion.
 34. The method ofclaim 33, wherein said compensating step includes the step of combininga quantity to said frame of digital data.
 35. A system, comprising atransmitter for transmitting digital data to a digital network, saidtransmitter configured to receive a quality feedback signal and tocompensate said digital data for rob bit signalling distortion based onsaid quality feedback signal, wherein said quality feedback signal isprovided by a receiver configured to receive said digital data and todetermine whether said digital data is distorted due to rob bitsignalling.
 36. The system of claim 35, wherein said quality feedbacksignal includes a pattern indicating which frames of said digital dataare subject to said rob bit signalling distortion.
 37. The system ofclaim 35, wherein said receiver is an analog modem.